Dun Laoghaire-Rathdown County Council have issued a warning that nobody should bathe at White Rock beach in Killiney due to high levels of E Coli bacteria contamination.
Water samples were taken on Monday last, and they were found to be heavily polluted with E Coli, which can cause disease in humans.
The source of the contamination is not yet known, and a spokesman for the council said that an investigation is underway.